Silicon carbide (SiC) single crystal is a promising semiconductor material that has a wide bandgap, high thermal conductivity, a high insulating electric field, and a high saturated electron velocity. Because of these characteristics, a semiconductor device produced of silicon carbide single crystal is able to operate at high speed and a high output level at a high operating temperature, so such semiconductor devices are promising for use in onboard power devices and energy devices, for example.
Methods conventionally known for growing silicon carbide single crystals include sublimation, the Atchison process, and liquid phase growth. Sublimation is a method in which SiC is used as the raw material, which is heated and sublimated to precipitate single crystal on a low-temperature component. The Atchison process involves reacting carbon and silica at a high temperature. Liquid phase growth is a method in which a silicon compound is dissolved in a carbon crucible, carbon and silicon are reacted at a high temperature, and single crystal is precipitated. However, each of these conventional methods had its problems, as discussed below. First of all, a problem that these methods have in common is that a high temperature is required for crystal growth. In addition, a problem in the quality of the resulting crystals with sublimation is that the resulting single crystals include numerous micropipes, stacking faults, and so forth. That is, in the course being sublimated, the raw material becomes silicon, SiC2, and Si2C and vaporizes, and it is difficult to control the partial pressures of these gases and keep them at the stoichiometric composition, and this is believed to be the cause of the above-mentioned defects. Also, with liquid phase growth, it is difficult to grow large crystals because of the small amount of carbon that dissolves into the silicon melt.
In an effort to solve the above problems, methods have been reported in recent years in which SiC single crystal is produced by a liquid phase growth method in which a melt is obtained by melting silicon, carbon, and a transition metal, and a seed crystal is brought into contact with this melt (see, for example, Patent Documents 1, 2, and 3). With these methods, a raw material with a composition of Si0.8Ti0.2 is put into a graphite crucible, the crucible is heated to 1850° C. in an argon atmosphere under atmospheric pressure to dissolve the raw material, and the temperature then is held at 1850° C. for 5 hours so that the graphite will dissolve into this melt. After this, a 6H—SiC seed crystal is immersed in the melt, the melt is cooled to 1650° C. at a rate of 0.5° C./min, and crystal is grown. It has been reported that SiC crystal with a thickness of 732 μm was formed with this method. Nevertheless, even with this method there is the problem in that a high temperature is required for crystal growth. Specifically, since the melting point of silicon is 1414° C., the melting point of carbon is 3500° C., the melting point of titanium is 1675° C., and the melting point of SiC is 2545° C., the temperature must be at least 1700° C. In particular, when a transition metal such as titanium is used, growing crystals at low temperature is difficult because of how high these melting points are. Another method that has been reported is one in which 3C—SiC single crystal is produced by a liquid phase growth method in which SiC is used as the raw material and a crystal is grown (Patent Document 4), but this requires treatment at a high temperature to obtain single crystal of high quality. Meanwhile, it is generally believed that for a high-quality SiC single crystal substrate to be produced at a low cost, the single crystal must be produced under low temperature conditions of 1500° C. or lower.
Patent Document 1: JP 2000-264790A
Patent Document 2: JP 2002-356397A
Patent Document 3: JP 2004-2173A
Patent Document 4: U.S. Pat. No. 4,349,407